1. Technical Field
The present disclosure relates to carbon nanotubes and more particularly to aligned carbon nanotubes. According to the present disclosure, conducting liquid crystal polymers are used to disperse and align the carbon nanotubes. The aligned carbon nanotubes of this disclosure can be used in the field of packaging of electronic devices, and more specifically as thermal interface materials for thermal management and cooling of semiconductor devices.
2. Description of Related Art
Thermal management is of great importance to the operation of electronic devices. Thermal management is especially important in the operation of semiconductors as factors such as increasing operating frequencies push power consumption, and therefore heat generation, to the limits of the cooling capacity of traditional passive, air-cooled, heat sink technology. The power density (W/cm2) in semiconductor devices continues to increase as the circuit density and operating frequency increase. Thermal management includes the skill of dissipating the heat generated by an electronic device away from the device and allowing the generated heat to disperse to its surroundings, while maintaining the semiconductor device at as low a temperature as possible. Insufficient transfer of heat away from an electronic device can result in performance and reliability degradation of that device or circuit due to an unacceptably high operating temperature.
Typical thermal management solutions use some combination of aluminum or copper heat sinks, fans, thermal spreaders/heat pipes, and thermal pastes or adhesives (TIMS) to form a low thermal resistance path between the semiconductor chip and the ambient.
In order to achieve an acceptable operating temperature for the chip, it is necessary to minimize the total thermal resistance (° C./W) from the chip to the ambient. Thermal interface material (TIM) which is the interface media between chip and heat spreader or heat sink is one of the bottle necks in thermal management. The TIM layers provide mechanical compliance to relieve the thermal expansion mismatch stress between the components that are constructed of different materials, but they also represent a significant portion of the total thermal resistance.
Thermal paste is one of the most common thermal interface material. It is a thick paste of conducting particles in oil or other low thermal conductivity media. Due to the interface resistance, the thermal conductivity of the paste is in the order of 1 W/mK. Thermal adhesive is another type of thermal interface material. The advantage of the thermal adhesive is that it serves both functions of thermal conduction and adhesion. Silicone and thermal epoxy are commonly used thermal adhesives. Metal or ceramic fillers are dispersed within a polymer binder to increase the thermal conductivity. Typical fillers are aluminum, silver, aluminum nitride, boron nitride, magnesium oxide, and zinc oxide. The typical polymer binder materials include silicone, urethane, thermoplastic rubber, and other elastomers. However, the thermal conductivity of the thermal adhesives are less than a few tens of W/mK. For instance see U.S. Pat. Nos. 5,781,412 and 5,213,868.
To further reduce the thermal resistance, a thin and high thermal conductivity material is needed. Carbon nanotubes (CNT) have very high thermal conductivity of >2000 W/mK. Many attempts of using CNT as fillers for thermal interface material have been made. It has been reported that by mixing 1% (in volume) of CNT in liquid, e.g. oil, the thermal conductivity of the suspension is 2.5 times of the thermal conductivity of the pure oil. However, further increasing the CNT concentration, only results in increasing the thermal conductivity minimally. It is believed that the entanglement of the CNT at higher concentrations and the high contact resistance between the tubes are the part of the cause.
Dissolution of CNT has been studied by Hadden (see U.S. Pat. No. 6,368,569 B1 and Science 282, pgs. 95-98, 1998). A self-assembled poly-domain nematic phase of the high concentration CNT in super acid solution has been obtained by Davis (see Macromolecules 37 pgs. 154-160, 2004). Vertically growing CNT on a substrate vertically by CVD has been demonstrated by Li et al (see Science 274, pg. 1701, 1996). However, the process is expensive and it is difficult to achieve on larger substrates.